In the prior art in communication networks of transmission lines a problem has always existed when branching of any sort was necessary. Essentially, all branching problems can be reduced to repetitions of the simple two-pronged branch represented by FIG. 1. Here, the stem and one of the prongs represent a main (or trunk) line, and the other prong represents a branch line to a wayside destination. Typical examples are: (1) a digital cable branching a few feet in distance from a computer-driven main cable to one of several peripherals stationed along the main cable, and (2) a main-line television cable with a branch line of several tens of feet in length leading to a customer's house. (Digital distributions systems of the type described in (1) are often called "party-line systems"). Such branching systems are basically troublesome. If the branch lines are terminated with the characteristic impedances of the cables, each such branch robs the trunk line of half of its power (or 29.3 percent of its voltage), and if the branch is terminated with any other impedance (or is left open-ended with an infinite impedance), the resulting transients ("ringing") caused by repeated reflections result in severe distortions of the signals carried by the cables.
In the U.S. Pat. No. 3,710,282 by Seinecke, issued Jan. 9, 1973, a considerable improvement ws made on the above problem in the case of branch line loads that are predominantly capacitative in nature. In Seinecke's invention, a resistor -- equal to the "wave resistance" (characteristic impedance) of the branch line minus half of the wave resistance of the main transmission line -- was inserted in series with the branch line adjacent to its junction with the main line.
In most of the prior art, the branching network is assumed to be carrying only CW (constant wave) signals consisting of a carrier wave with only small variations in wavelength due to modulation. In general, the lengths and placements of the branches are stated in terms of fractions or integral numbers of wavelengths of the contained signal insofar as they apply to lengths of the branch line or the placement of the branches with respect to the terminus of the main line. The branches may be open or may be terminated with a variety of impedances. The characteristic impedances of the branch lines are generally equal to that of the main line, but may rarely be higher. In effect, the branch lines in the typical case become stubs.
There is additionally a general practice of stubbing which has no intent of providing branch signal lines, but only of modifying the behavior of the signal of the main line. From one to three stubs may be involved, often open-ended, and the stub lengths and mainline placements are invariably stated in terms of fractions or integral numbers of the wavelength of the contained signal. In one case (U.S. Pat. No. 1,933,669), stubs are added to branches, but they are still stated in terms of the carrier signal wavelength. None of these cases is suitable for pulsed or broad-band signals without incurring excessive losses due to transients and signal distortions.
For pulsed signals, there may exist a rare usage of a pair of stubs of nearly, but not exactly, equal lengths for the purpose of performing some act of discrimination on closely-spaced pulses that are generated in the field of nuclear electronics.
In general for pulsed signals, Bewley lattices (which are "bounce diagrams" of time and distance parameters) show that reflections in random-length, single, open-ended stubs are repeated. They die exponentially, sometimes slowly. Each new round trip in the stub is a source of new perturbations on the incoming and outgoing main line. Similar troubles occur if the stubs are closed. Even if the stubs are terminated in characteristic impedances and are either short or long, becoming branch lines, the base junctions of the stubs are sources of unwanted reflections and serious loss of forward power in the main line.
The prior art in switching trees consists of two types. The first type is composed of two-pronged switching elements, providing a one-to-two series fan-out, or can be reversed to a two-to-one series fan-in. In multiple-outlet switching trees it contains n stages in series for a maximum of 2.sup.n possible outlets, of which one outlet is selected by choosing the appropriate switches through the tree to provide the desired path. This type of tree has two defects. First, it maximizes the number of switches and, consequently, the amount of contact resistance in the selected path. Second, it has n single stubs along its path, each of which is a source of multiple reflection whose ringing severely distorts the signal in the path. Parasitic capacitance on the path is minimized, however. The second type of switching tree is hardly a tree at all, but is commonly called a "multiplexer." It has only one stage in which the inlet goes directly to the outlets in star fashion. The desired path is usually selected remotely. For, say, 2.sup.n outlets in the multiplexer, the desired path has 2.sup.n distortion-producing stubs, all at one junction. Although contact resistance is minimized, being only one contact, the 2.sup.n stubs maximize parasitic capacitance. Thus a 32-branch tree has five levels of switching elements. The second type is a multiplex type of switch which has only one level of fan-out or fan-in going, say, from 1 to 32 or 32 to 1. In-between series are not used. While the first type minimizes the parasitic capacitance per path, it maximizes the series contact resistance. With the multiplex series it is the reverse. When either type of switching tree is used on transmission lines, however, a significant distortion is imposed on the signal (either pulsed or CW) which travels through the line. This distortion arises from the reflections at the branch points and the open stubs that inherently occur in the switching trees.